Jim's Corner Blog

Big, Bigger, and Biggest Backyard Telescopes

This month’s column is a cautionary discussion against aperture fever. Whenever you get the overwhelming desire for a bigger telescope, heed these warnings.

A look at the marketplace, major star parties (such as Stellafane, RTMC and the Winter Star Party) and in a small number of amateur astronomer’s backyards, there are telescopes ranging in size from 10” or larger, encompassing two families of large telescopes, the large Dobsonian and the SCT.

The most common large telescopes for visual use are the 10” to 12” Dobsonian telescopes and the 10” to 12” SCTs. 10” Dobs and 10”-11” SCTs are basically the practical upper limit to portable personal telescopes. Larger than these sizes results in telescopes that are difficult to transport and multiple people to assemble at a remote site.

Occasionally, a 16” Dobsonian shows up as a commercial product. There are off-the-shelf commercially available 13” or larger Dobs and 14”-16” Schmidt-Cassegrain telescopes that are available from both the “Big Two” manufacturers and some smaller suppliers. This category of large backyard telescopes are at best transportable, although the size and bulk makes them somewhat cumbersome and sometimes needing more than one person to set up. There are no commercially available refractors in this aperture grouping. For the record, any 6” or larger refractor becomes as cumbersome to transport as a 16” SCT because of the large mounting requirements.

Jim’s Celestron 11” SCT (James Chen)

Then there are the “Ferrari’” and “Rolls Royce” of visual use amateur telescopes. Included in this grouping are the truss construction large Dobsonians of apertures from 13” to 24” or larger. These telescopes are made by artisans at small telescope companies and are made to order. Again, there are no commercially available refractors in this aperture size grouping.

Briefly, there was an attempt by one telescope vendor, Orion, to market 36”, 42”, and 50” Dobsonians! Orion tried to premier their super colossal Dob at NEAF a few years ago, but were unable to get the telescope assembled for the expo. The plan to market these massive monster telescopes was eventually dropped.

7”, 24”, and under the cover a 30” Dobsonian at the Winter Star Party 2014 (James Chen)

Included in this super large telescope family are the 14” and 16” SCTs manufactured by the two major telescope manufacturers. The telescopes at Frederick County Middle School are 16” Meade LX-200 SCTs. One of our former members, John Hershey, is well remembered for his 14 inch Celestron SCT which he would bring to every public outreach that our club held. In order to assemble his telescope at a star party, he had a special mechanical lift that enabled him to hoist the OTA onto his mount. That’s dedication!!!

These large SCTs have all the bells-and-whistles, including GoTo capability and are astro-imaging capable with added accessories. Available with heavy duty field tripods, these superstars of the SCT world are best considered as transportable by a small number of people rather than portable and useable by a single person (with the exception of John). Ideally, these large SCTs lend themselves best to permanent setups in an observatory. There are some rare custom made refractors that equal or exceed 12”, and these are normally mounted under an observatory dome. These telescopes are generally not portable. In military terms, large SCTs are transportable (but only barely). They often require more than one person to set up and disassemble. Our club’s experience with the FCMS 16” SCTs is a perfect case study. We needed four people to lift the telescopes onto the mount drive assembly!

The views through these massive marvels are fantastic, definitely evoking a “WOW” factor through the eyepiece. The increased light gathering enables the observer to see fine tendrils and filaments of nebulosity in emission nebulae, extended views of galaxies and revealing galactic dust lanes, and layer upon layer of starry diamonds within globular clusters.

The largest current record holder for a large backyard telescope is a 70” Dob located in California, built from a military surplus spy satellite mirror. Historically, William Parsons, The Third Earl of Ross, hold the record for the largest amateur telescope, the 72 inch The Leviathan of Parsonstowm.

But there are downsides to the larger telescopes. Many are expensive, with the larger telescopes requiring the additional purchase of a truck or van in order to transport the telescope to a remote site. The larger Dobsonians require 8 foot or 10 foot ladders to reach the eyepiece. Many are difficult to setup on a routine basis. Most do not qualify as “grab-and-go” telescopes.

Always remember the telescope salesman’s Golden Rule, the smaller telescope that gets used a lot sees more than the bulkier, difficult to use larger telescope seldom used. That’s why the best selling sizes of telescopes are the 4” refractor, the 8” SCT, and the 10” Dob.

Jim's Corner Blog

Musings of a Backyard Astronomer

A Historical View of Telescopes

From a historical viewpoint, today’s commercially available telescopes and eyepieces are technological marvels that the great astronomers of history would have loved using. Who knows what further discoveries that William Herschel or Charles Messier would have made with today’s high quality and sophisticated telescopes and wide field eyepieces.

Charles Messier was a French astronomer of the middle to late 1700’s to early 1800’s, who during his lifetime was more noted as a comet hunter than a deep sky observer. As a result of stumbling upon diffuse “fuzzy” objects that did not move in the sky, Messier, the comet hunter, began compiling a list of fixed diffuse objects in the night sky which could be mistaken for comets. Ironically, in today’s world, Charles Messier is best remembered for his Messier catalog of over 110 deep sky objects rather than for his 13 comets that were discovered during his lifetime.

Messier’s favorite instrument was a 7.5” aperture Gregorian reflector with a 3 foot focal length, with a fixed magnification of 104x. With its speculum metal mirrors, it has been calculated that the effective aperture of this instrument was equivalent to a 3.5-inch refractor. Even worse was the situation for the old 8-inch Newtonian reflector he occasionally used, which again was equipped with speculum mirrors and could only achieve the performance of a modern 2.5” refractor. Later he preferred to use several 3.5” achromatic refractors, with focal lengths of about 3.5 feet, and magnifying 120 times. He selected to use these scopes because they were the most easily accessible instruments for him. All of Messier’s telescopes appeared to have fixed magnifications. Apparently, telescopes of his time did not have interchangeable eyepieces.
All of Messier’s instruments are not as capable as a modern 4” refractor or 8” Schmidt-Cassegrain telescope. Backyard astronomers of today can observe all the objects of the Messier catalog using a 3” or 4” refractor in dark skies.

The story of William Herschel, his sister Caroline, and his son John, and their contributions to the science of astronomy is well documented. Sir William Herschel, born in Germany as Frederich Wilhelm Herschel in 1738, escaped the French occupation of Hanover, Germany and immigrated to England in 1757. A music teacher by trade, he became obsessed with astronomy. Telescopes were not a common place item in England of the 1700’s, so Herschel resolved to learn how to build his own. With his sister Caroline helping, William Herschel began experimenting with grinding mirrors and building reflecting telescopes.

Using innovations, such as the parabolic mirror and a new reflective coating alloy that used an increased mixture of copper in its formulation, Herschel built his first telescope, a 6” aperture with a 7-foot focal length. This initial foray into telescope making was easily capable of seeing Saturn’s rings.

But William knew he could build a bigger mirror, and followed that with a telescope with a mirror diameter of 9 inches. As the mirror size increased, so did the focal length of the telescope, with a focal length growing to 10 feet in length.

His next instrument proved to be his life-long favorite telescope. An 18” aperture with a focal length of 30′, an extremely long instrument by today’s standards. It took him three tries before successfully completing the fabrication of this 18” mirror, with the first attempt ending with a cracked mirror and the second with a molten metal mess.
He went on to complete the construction of a massive 48” instrument, with a focal length of 40 feet! Although certainly the greatest light gathering could be achieved with this instrument, it proved to be very cumbersome to operate, requiring at least two assistants to operate the massive telescope. This telescope and the 18” telescope were not of the commonly accepted Newtonian reflector design. Herschel tilted the primary mirror so that the focus would occur slightly to the side of the telescope. The observer had to be on a platform and lean over the telescope to see the image. This meant that the difficult-to-make-flat diagonal normally used in a Newtonian design could be eliminated. In the case of the 48”, it meant that Herschel was hanging precariously over the end of the telescope tube as much as 40 feet in the air! Few “Herschelian” telescopes exist today, and their design has been left to amateur telescope makers to attempt the challenge of building.
With these telescopes, William Herschel was able to discover the planet Uranus. He and his sister Caroline went onto discover over 2,500 deep sky objects and 848 double stars.
Telescope technology has advanced far beyond the equipment that Messier and Herschel used. Modern telescopes, with advanced telescope optics, advanced coatings, modern eyepiece designs, and the mechanical and electronic innovations of telescope mounting systems, can out-perform any of the classic telescopes of Messier’s or Herschel’s era. The average backyard astronomer with today’s high quality 4” refractor, 8” Schmidt-Cassegrain, or 10” Dobsonian can, with dark skies and due diligence, observe all of Messier’s and most of Herschel’s discoveries.

Jim's Corner Blog

The Leviathan of Parsonstown

M51 was discovered by Charles Messier on October 13, 1773. Messier apparently recognized only the larger portion. It was left to Pierre Méchain on March 21, 1781 to recognize the smaller portion. Messier described it as “very faint nebula, without stars”. Mechain then reported it as “It is double, each has a bright center, which are separated 4’35”. The two “atmospheres” touch each other, the one is even fainter than the other.” Hence M51 is comprised of M51a and M51b, and has two NGC identifications, NGC 5194 and NGC 5195.

It was not until William Parsons, 3rd Earl of Rosse, using the famous 72” speculum reflector The Leviathan of Parsonstown at Birr Castle Ireland , observed and drew the now recognizable spiral structure of M51.

William Parsons, 3rd Earl of Rosse sketch of the spiral structure of M51

The 72-inch Leviathan telescope, still regarded as the largest amateur telescope in history, replaced a 36-inch telescope that Parsons had built previously. Parsons had to invent many of the techniques he used for constructing the Leviathan, both because its size was without precedent and because earlier telescope builders had guarded their secrets or had simply failed to publish their methods. The Leviathan of Parsonstown was considered the scientific, technical, and architectural achievement of its time, and images of it were circulated widely within the British commonwealth. Building of the Leviathan began in 1842 and it was first used in 1845. It was the world’s largest telescope, in terms of aperture size, until the early 20th century when the building of the 100-inch Hooker telescope at Mt. Wilson ascended to the throne of World’s Largest Telescope. William Parson was the last amateur astronomer to build and own the world’s largest telescope. Since then the title of world’s largest telescope has gone to instruments built and operated by educational or scientific institutions.

The Leviathan was not a perfect telescope. It was awkwardly mounted and was labor intensive to operate and use. It had a limited alt-az mounting (no clock drive here!) capable of only views 7º to either side of the meridian. It used a speculum mirror that tarnished easily. As a result, two speculum mirrors were created, so that one could be used for observations while an army of men on a monthly basis would disassemble the telescope, change out the mirrors, realign the newly installed optics while re-polishing the other mirror.

The poor Irish weather often interfered with the Leviathan’s operational use. But when the weather was good, William Parsons made scientific history with his Leviathan of Parsonstown.

The largest amateur telescope today is a 70-inch Dobsonian, constructed from a U.S. Government surplus spy satellite mirror and parts from Lowe’s and Home Depot, built by Mike Clements and currently housed in the Salt Lake City area of Utah.